The use in liquid crystal materials of 4-alkyl and 4-alkoxy-4"-cyano terphenyls is described in GB 1433130, among many other publications and liquid crystal materials containing them are widely used. GB1433130 suggests the possibility of one of the three phenyl rings being laterally substituted, suggested substituents being the bulky methyl group or an unspecified halogen. No example of any such laterally substituted terphenyl is disclosed in GB1433130, no method for their preparation is given, and no preference for any particular halogen substituent is expressed.
GB2039937A describes liquid crystal materials which contain at least one laterally substituted 4-alkyl-4"-cyanoterphenyl of general structure: ##STR1## where R is alkyl, one of X.sub.1 or X.sub.2 being hydrogen and the other being methyl or a halogen preferably the bulky chlorine. A complex 6 step method for preparation of these terphenyls is described in GB2039937A, but no example of a terphenyl having X.sub.1, or X.sub.2 as a halogen is described, the implication being that none were made.
Mol. Cryst. Liq. Cryst. (1985) 123 169-177 describes 2 and 2' fluorosubstituted 4-alkyl-4'-cyanobiphenyls, but the data presented therein suggests that lateral fluorination increases the melting point of these compounds, and that their solubility in liquid crystal materials is low.
Laterally fluoro-substituted terphenyls of general structure: ##STR2## where R.sub.1 and R.sub.2 are alkyl or alkoxy are known and examples are given in EP-A-0132377. The preparation method described therein cannot easily be applied to the preparation of a terminally cyano substituted terphenyl.
The present invention has identified a class of terphenyls which on the basis of the prior art discussed above appear to be newly prepared and which show advantageous properties. A novel process for their synthesis which for the first time makes them also commercially viable has also been discovered.